    The following abbreviations are herewith defined.    BSC Base Station Controller    BTS Base Transceiver Station    CN Core Network    CRS Cell Re-Selection    DL Down Link (to the MS)    EDGE Enhanced Data rate for Global Evolution    EGPRS Enhanced General Packet Radio Service    GERAN GSM/EDGE Radio Access Network    GGSN Gateway GPRS Support Node    GPRS General Packet Radio Service    GMM GPRS Mobility Management    GSM Global System for Mobile Communications    GSN GPRS Support Node    HO Handover    LLC Logical Link Control    MAC Medium Access Control    MS Mobile Station    MSC Mobile Switching Center    PDP Packet Data Protocol    PDU Packet Data Unit    RLC Radio Link Control    RNC Radio Network Controller    SAPI Service Access Point Indicator    SGSN Serving GPRS Support Node    TBF Temporary Block Flow    TLLI Temporary Logical Link Identity    UL Uplink (from the MS)    UMTS Universal Mobile Telecommunications System    URA User (or UTRAN) Registration Area    UTRAN Universal Terrestrial Radio Access Network
Reference can also be made to 3GPP TR 21.905, V4.4.0 (2001-10), Third Generation Partnership Project; Technical Specification Group Services and System Aspects; Vocabulary for 3GPP Specifications (Release 4), as well as to ETSI TR 101 748, V8.0.0 (2000-05), Digital cellular telecommunications system (Phase 2+); Abbreviations and acronyms (GSM 01.04 version 8.0.0 release 1999).
In general, when the MS (Mobile Station) changes from one cell to another in the GPRS (General Packet Radio Service) system any ongoing TBFs in the old cell are terminated and the new cell is entered. The cell change decision can be made by the MS or by the wireless network. In the case where the network makes the cell change decision, the network sends a Packet Cell Change Order message to the MS. The MS leaves the old cell and enters the new cell almost as if the MS had itself made the cell change decision.
In GPRS there is no handover as in the GSM system, meaning that the radio connections (TBFs) are not maintained during the cell change. When the MS enters the new cell any ongoing TBFs in the old cell are released and re-established in the new cell. A handover procedure in which radio resources are pre-allocated in the new cell is not currently defined in the GPRS specification.
When the GPRS MS is operating in a GMM (GPRS Mobility Management) READY state, and when the MS enters the new cell, the MS is expected to send an LLC PDU (Logical Link Control Packet Data Unit) to the SGSN (Serving GPRS Support Node). Based on the LLC PDU the SGSN is able to determine in which cell the MS is located. As such, the SGSN is not required to initiate a Paging procedure when sending LLC PDUs to the MS.
In the GPRS system, when a MS operating in the GMM READY state changes to a new cell (either MS initiated or network controlled) that is associated with the same routing area as the previous cell, the following process is executed:    (i) the MS determines that it will change to the new cell;    (ii) the MS terminates all TBFs (Temporary Block Flows) in the previous cell, meaning that an ongoing data transfer is terminated abnormally (a TBF is a unidirectional radio connection between MS and the network, where there may be an UL (Uplink) and/or a DL (Downlink) TBF established at any given time);    (iii) the MS enters the new cell;    (iv) the GMM or LLC of the MS (depending on the implementation) requests the LLC to send a LLC PDU to the network;    (v) the LLC of the MS sends the LLC PDU to the RLC/MAC of the MS;    (vi) when the RLC/MAC of the MS has obtained the parameters related to packet access from a System Information message being broadcast in the new cell, or has obtained these parameters via some other means (such as from the previous cell), the RLC/MAC of the MS initiates an UL TBF establishment procedure;    (vii) when the UL TBF is established, the RLC/MAC sends the LLC PDU to the network; and    (viii) the SGSN determines from the received LLC PDU the new location of the MS.
A publication entitled Digital Cellular Telecommunications System (Phase 2+); General Packet Radio Service (GPRS); Mobile Station (MS)—Base Station System (BSS) interface; Radio Link Control/Medium Access Control (RLC/MAC) protocol; (GSM 04.60 version 6.8.0 Release 1997) requires that the received (and segmented) LLC PDUs be put into RLC data blocks in the same order as they are received from higher layers. This means that if there was an ongoing data transfer in the previous cell, the RLC of the MS may have several untransmitted LLC PDUs in its transmit buffer that are to be transmitted to the network in the new cell. The above-mentioned cell change LLC PDU that was initiated by the GMM is thus placed in the last available entry of the transmit queue (i.e., behind any untransmitted LLC PDUs). Before the SGSN receives the first LLC PDU from the MS via the new cell, the SGSN sends all DL LLC PDUs to the previous cell. If the LLC is operating in an unacknowledged (UNACK) mode, the DL LLC PDUs destined to be transmitted to the previous cell are discarded because the network can not contact the MS via the previous cell. If the LLC is operating in an acknowledge (ACK) mode, those DL LLC PDUs destined to be transmitted to the previous cell must be retransmitted to the new cell, which takes time and unnecessarily wastes network resources.
Two examples are now provided to further clarify the current state of the prior art.
Example 1, ACK RLC mode
Assume that the RLC of the MS has one 1500 octet LLC PDU in its transmit buffer when the MS changes from the previous cell to the new cell. In GPRS the radio resources may be shared between several MSs. If the MS is required to share TSs (time slots) that are assigned to its UL TBF with other MSs, the MS may not receive permission to send in a timely manner, and it may require a significant amount of time to transmit the LLC PDU. Furthermore, if some RLC data blocks are lost they must be retransmitted, which requires even more time. As a result, the SGSN may transmit a large number of DL LLC PDUs to the previous cell before the SGSN learns of the new cell location of the MS.
Example 2, UNACK RLC mode
Assume as in example 1 that the RLC of the MS has one 1500 octet LLC PDU in its transmit buffer when the MS changes from the previous cell to the new cell, and is also required to share radio resources. In the UNACK case if some RLC data blocks are lost, the RLC on the network side sets zero octets into the LLC PDU to replace the missing data octets. If the LLC in the SGSN calculates a CRC for the LLC PDUs, one missing RLC data block means that the CRC calculation fails. Since it takes some tens of RLC data blocks (depending on the channel coding scheme) to transmit one 1500 LLC PDU, the probability of one block going missing for whatever reason is increased. If the LLC PDU CRC calculation fails in the SGSN, the LLC in the SGSN discards the LLC PDU and the SGSN does not update the new cell location of the MS.
The presence of missing user data packets can result in detrimental side effects for the user, as MS applications that rely on receiving the user data packets can malfunction, or even cease to function.
As can be appreciated, there are significant problems inherent in the prior art approach to the MS changing cells in the GPRS.